1. Field of the Invention
This invention is concerned with a process for converting synthesis gas (H.sub.2 and CO) to dimethyl ether. It is also concerned with a novel catalyst composition for effecting the conversion reaction.
2. Discussion of the Prior Art
There are at present two major routes for effecting the conversion of coal via synthesis gas to liquid fuels comprising the well publicized Fischer-Tropsch process and a more recently developed methanol to gasoline process such as provided in U.S. Pat. No. 3,928,483, issued Dec. 23, 1975. The Fischer-Tropsch process produces a wide range of C.sub.1 to C.sub.50 products comprising gases, liquid hydrocarbons, oxygenates and water.
Common to each of the above processes is the overriding influence of the capital cost of synthesis gas (H.sub.2 +CO) production. This varies with gasifier design, which is in turn influenced by coal properties and product enthalpy. A recent review of gasifier technology has identified some high efficiency gasifiers which of necessity produce synthesis gas having relatively low ratios of H.sub.2 /CO resulting from utilizing low ratios of steam and oxygen in the gasification operation. Thus, the most recent advanced gasifiers operate at temperatures requiring relatively low product recovery temperatures and low steam to carbon ratios which translate into high thermal efficiency. The "syngas" produced under these conditions may have a ratio equal to 1 and more usually, for the highest efficiency gasifiers, a ratio of less than 1 and within the range of about 0.4 to about 0.7. Such low ratio syngas cannot be directly utilized by the present day conventional Fischer-Tropsch operations and methanol synthesis processes, both of which require H.sub.2 /CO ratio equal to or greater than 2. Any external water-gas shift operation to increase a low ratio syngas of 1 or less up to 2 or more would substantially cancel any gains in efficiency achieved by the most advanced high efficiency gasifiers.
A principal advantage to be gained in producing dimethyl ether (DME) directly from syngas is that it has been found that this compound can be readily converted to gasoline range hydrocarbons using a special class of crystalline porotectosilicates, represented by ZSM-5 crystalline zeolite such as discussed in U.S. Pat. No. 3,928,483, wherein the formed methanol is dehydrated and the ether product thereof is converted over said ZSM-5 crystalline zeolite.
Prior to our present discovery, two patents dealing with synthesizing DME directly were known. These include British Pat. No. 278,353 (1926) to F. R. Bechowsky and German Offen. No. 2362944 (1974) to G. Pagani. The British patent claims a process for producing DME by contacting synthesis gas (H.sub.2 +CO) with a hydrogenating catalyst and a dehydrating catalyst at elevated temperatures and pressure. In this patent, DME synthesis was obtained in the absence of the known shift reaction. In the second or German patent, the catalyst comprised a methanol synthesis component and a dehydrating component. In one example, a gas with H.sub.2 /CO ratio of 0.86 was contacted with a catalyst comprising Cu/Zn/Cr, in an atomic ratio of 82/16/4 and supported on alumina, at 482.degree. F. and 1422 psia. The conversion of the syngas was 77%. The exit gas contained 24.2% DME, 0.91% MeOH, 27.3% CO.sub.2, 0.41% H.sub.2, 0.54% CH.sub.4, with the balance being H.sub.2, CO and N.sub.2.
In more recent work, Sherwin and Blum reported in an Interim Report for May 1978 under the title "Liquid Phase Methanol", prepared for Electric Power Research Institute, Palo Alto, Calif., the attempt to produce DME by adding gamma-alumina and 13X molecular sieve to a slurry reactor system containing a commercial methanol synthesis catalyst. At 446.degree. F.-572.degree. F., 515-1015 psia and 2015-6915 GHSV, only traces of DME were observed. A catalyst comprising Cu/Zn/Cr, in an atomic ratio of 6/3/1 impregnated on Davidson 980 SiO.sub.2 /Al.sub.2 O.sub.3, produced trace amounts of DME at 446.degree. F., 1015 psia and 2000 GHSV. The feed charged in the above experiments contained 50% H.sub.2, 25% CO, 10% CO.sub.2 and 15% CH.sub.4.